code: def test_list_sketch(self):
      list_data = [
          [],
          [1, 2],
          [1, 2],
          ["a", "a", "a", "b"],
          [1, 1, 2],
          None,
      ]
      sa = SArray(list_data)
      self.__validate_nested_sketch_result(sa)
      sketch = sa.summary()
  
      self.assertEqual(sketch.num_unique(), 4)
      element_summary = sketch.element_summary()
      another_rep = list(
          itertools.chain.from_iterable(list(sa.dropna()))
      )
      self.__validate_sketch_result(
          element_summary, SArray(another_rep, str)
      )
  
      fi = sketch.frequent_items()
      self.assertEqual(len(fi), 4)
      self.assertEqual((fi["[1,2]"]), 2)
      self.assertEqual((fi['["a","a","a","b"]']), 1)
  
imports: from __future__ import print_function
  from __future__ import division
  from __future__ import absolute_import
  from ..data_structures.sarray import SArray
  import pandas
  import numpy
  import unittest
  import random
  import copy
  import os
  import math
  import array
  import time
  import itertools
  import pytest
label_desc: Use image node_id 6 for calling the SArraySketchTest obj's underlying member method code with example usage: obj.test_list_sketch() without return types